This invention relates to the use of coal fines produced as a result of mechanical breakage and crushing that occurs in a coal wash plant to make useful briquettes. Fines as used herein generally refer to those materials which pass through a 48 mesh Tyler sieve screen.
The process of briquetting consists of applying pressure to a mass of coal particles, with or without the addition of a binder to form a compact agglomerate. It has a wide application industrially and is not confined to coal alone. An understanding of the process necessarily involves a knowledge of the cohesive forces between solids, the adhesion of binders, and the rheological behavior of particular masses and agglomerates.
The briquetting process, in essence, is one in which coal particles, typically are covered with an adhesive and pressed into molds. The method entails the basic steps of drying and grinding the coal, mixing it with a binder, heating the mixture, pressing the mixture into molds, and cooling and loading the briquettes. These steps are generally known in the briquetting industry.
Since they are well known, they will not be described in detail herein, except in general terms. For further details see Chemistry of Coal Utilization, Supplementary Volume H. H. Lowry, Editor, published by John Wiley & Sons, Chapter 16, pages 675 through 753, the disclosure of which is incorporated herein by reference.
Binders are commonly used, as the name implies, in order to provide greater structural integrity of the briquettes formed during processing.
The two most commonly used binders are pitch and bitumen. The typical range of binder addition is within the order of from about 5% to about 18% by weight, and in some instances as low as 1% to 2%. In Europe, pitch is the most common binder material. It is the residue left when coal tar is distilled. It is composed of a very large number of chemicals ranging in complexity from the fairly simple anthracene compounds to others of comparatively high molecular weight. In its role as a binder, the chemical composition of the pitch is of far less consequence than the physical properties. In producing a briquetting pitch the distillation of coal tar is terminated at such a temperature as will leave a residue which is medium soft. Coal tar pitch is quite expensive, and currently may cost as much as $180 per ton.
Besides expense, coal tar pitch is also known to be carcinogenic to those handling it. Some evidence indicates that it may cause epithelial cancer.
It can therefore be seen that the development of a briquette which will maintain its structural integrity without the use of a pitch binder would have readily apparent advantages.
In the United States, pitch is not commonly used as a binder, but bitumen is used in its place. Bitumen is a material derived from petroleum oil as a distillation residue. It is commonly referred to as asphalt. However, bitumen possesses most of the virtues and faults of coal tar pitch. It has good binding properties but it is expensive, smokey and unpleasant to handle. It therefore would be desirable to develop a process of briquetting which eliminates the need for bitumen binder, as well as pitch binders.
Of course, as heretofore mentioned, the purpose of adding binders in the overall briquetting process is to make briquettes which will maintain their structural integrity during storage, shipment and usage. Such briquettes must also maintain their integrity during "weathering" so that they still will be useful afterwards.
"Weathering" is simply subjection of the briquette to ordinary environmental conditions such as moisture and the like. In a laboratory, weathering tests are normally conducted by soaking the briquette in water for a period of time and then subjecting the briquette to friability testing. Typically the briquette's friability or in other words its structural integrity, is tested by a tumbler test, that is, it is placed in a tumbler, subjected to tumbling action and then the amount of breakage to fines is measured.
Satisfactory briquettes for commercial usage must maintain their integrity after weathering, since they are often subjected to weathering during normal usage.
Accordingly, the primary object of this invention is to prepare briquettes from coal fines, which eliminate the use and necessity for added binder materials such as coal tar pitch and bitumen; and, yet which will provide a briquette which is of good structural integrity, even under weathering conditions, and which eliminates the possible harmful effects which may occur from the use of traditional binders.
The method and manner of accomplishing these as well as other objectives of the invention will become apparent from the detailed description which follows.